In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. For example, the ink can be first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful to prepare lithographic printing plates typically comprise an imageable layer applied over the hydrophilic surface of a substrate. The imageable layer includes one or more radiation-sensitive components that can be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the non-imaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the element is considered as positive-working. Conversely, if the non-imaged regions are removed, the element is considered as negative-working. In each instance, the regions of the imageable layer (that is, the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Direct digital imaging has become increasingly important in the printing industry. Imageable elements for the preparation of lithographic printing plates have been developed for use with infrared lasers. Thermally imageable, multi-layer elements are described, for example, in U.S. Pat. No.6,294,311 (Shimazu et al.), U.S. Pat No. 6,352,812 (Shimazu et al.), U.S. Pat. No. 6,593,055 (Shimazu et al.), U.S. Pat. No. 6,352,811 (Patel et al.), and U.S. Pat. No.6,528,228 (Savariar-Hauck et al.), and U.S. Patent Application Publication 2004/0067432 A1 (Kitson et al.). U.S. Patent Application Publication 2005/0037280 (Loccufier et al.) describes heat-sensitive printing plate precursors that comprise a phenolic developer-soluble polymer and an infrared radiation absorbing agent in the same layer.
Additional positive-working thermally imageable elements are described and used for making lithographic printing plates using various developers in U.S. Pat. No. 6,200,727 (Urano et al.), U.S. Pat. No. 6,358,669 (Savariar-Hauck et al.), and U.S. Pat. No. 6,534,238 (Savariar-Hauck et al.). In some instances, such imageable elements are developed using low pH developers when the upper layer includes novolak resins and dissolution suppressing agents.
Single-layer, positive-working imageable elements are described for example, in U.S. Pat. No. 6,280,899 (Hoare et al.), U.S.Pat.No. 6,391,524 (Yates et al.), U.S.Pat. No. 6,485,890 (Hoare et al.), U.S.Pat. No. 6,558,869 (Hearson et al.), and U.S. Pat. No. 6,706,466 (Parsons et al.), and U.S. Patent Application Publication 2006/0130689 (Müller et al.).
Copending and commonly assigned, U.S. application Ser. No. 11/686,981 (filed Mar. 16, 2006 by Savariar-Hauck et al.) describes and claims a method of processing using low pH developers in which the processed elements contain certain phenolic resins in the upper layer. Other imaged elements developable in low pH developers are described in U.S. Pat. No. 6,555,291 (Savariar-Hauck).
Problem to be Solved
After thermal imaging, the imaged positive-working elements are developed to remove exposed regions of all layers to expose the hydrophilic substrate. There is a desire to use lower pH developers (pH 11 or less) as the non-imaged (non-exposed) regions are more hydrophilic and this enables better press restart properties with less toning problems. This is true because the substrate interlayer is not removed during processing. However, the use of lower pH developers can lead to considerable residue build up in the developer due to insufficient solubility of removed polymeric binders. Build up of sludge or residue causes a number of maintenance problems including the use of large filter units besides lowering the useful life of the developer. There remains a need to provide alternative means for using lower pH developers without a buildup of significant residue or sludge in the developers.